The characterisation of epitaxial layers of the dilute magnetic semiconductor Hg(_1-x)Mn(_x)Te

Abstract

This work is concerned with the characterisation of epitaxial (Hg,Mn)Te, and in particular the assessment of the structural and transport properties of the layers. Direct alloy growth (DAG) of (Hg,Mn)Te on buffered GaAs and unbuffered (Cd,Zn)Te substrates in a horizontal MOVPE reactor resulted in poor surface compositional and thickness uniformity. This was attributed to the differences in the pyrolysis rates of the Te and Mn precursors. Double crystal x-ray diffraction was used to determine accurately layer thickness, composition and quality. Analysis of the symmetric and asymmetric rocking curves revealed that for thickness over lµm (Hg,Mn)Te was almost 100% relaxed. A reduction in the dislocation density, as inferred from the x-ray rocking curve width, was observed with increasing layer thickness and was considered in terms of the Gay model. A sub-grain structure was seen using double crystal x-ray topography, where grain diameter decreased in a linear manner with inverse layer thickness. Triple axis x- ray diffraction revealed that tilts between the sub-grains were the primary cause of the rocking curve broadening. Hall and resistivity measurements were made in the temperature range 4K to 290K. The majority of the layers exhibited n-type behaviour, with carrier concentrations and mobilities dependent on layer thickness. Modelling of the temperature dependence of the carrier mobility was attempted for various lattice and impurity scattering mechanisms. Attempts were also made to explain the temperature dependence of the carrier concentration using a two band model and estimates for the bandgaps obtained from intrinsic Hall data. The interdiffused multilayer process (IMP) was then employed, resulting in layers with superior lateral compositional and thickness uniformities. Improved crystalline quality to that seen in comparable DAG layers was also obtained. The electron mobilities were higher in IMP lasers than incomparable DAG ones and analysis of the mobility data indicated that several scattering mechanisms were operative, including ionised and neutral impurity, optical phonon and piezoelectric (in Mn rich layers) scattering. Layers grown by IMP appeared to be more stable and changes in transport properties over a period of a few months were less pronounced than for comparable DAG layers.